Electrochemical studies with solid, stationary electrodes have usually employed noble metals (e.g. platinum, gold) and carbon. Advantages offered by these materials include easy fabrication into an electrode, large useful potential range, and good electrical conductivity and reproducibility.
Very often, mechanistic, kinetic and diagnostic studies are performed with small electrodes fashioned into rods which are insulated except for the cross-sectional area at one end of the rod (typical diameter of 3 mm). To increase reproducibility, the electrode is high polished to ensure a like surface from experiment to experiment.
Many battery systems employ carbons with surface areas of 60-1500 m.sup.2 /gram to construct porous cathodes. Binders (e.g. teflon or other polymers) are used to hold the carbon particles together. Direct comparison of porous carbon electrodes with glassy carbon or metal rod electrodes is difficult since porous carbon cathodes are necessarily large and thick (by comparison) in order to maintain mechanical integrity during and after fabrication. Rods of metal or glassy carbon as described above have surface areas nearly equal to the 2-dimensional geometric area one measures. Multiplying the area of glassy carbon to "scale up" surfaces to reflect that of various 3-dimensional porous carbons is not valid, especially since structures, end groups and properties vary between each manufacturer of carbon. A method of preparing a very thin, miniature porous carbon cathode on scale with glassy carbon rod electrodes would be desired so that carbons could be easily and rapidly ranked for electrochemical characteristics in small laboratory test cells rather than the present practice of preparing large glass laboratory cells or hermetically sealed cells. Further, elimination of binder materials would provide one less variable to be considered for comparison studies.
The study of electrochemically formed conducting polymers and their use in electrochemical systems as cathode materials has recently become of interest. Films can be directly polymerized onto conductive substrates (e.g. platinum). One attribute of polymer films (in the range of 1-20 .mu.m thick) is that the magnified surface is very rough, meaning the true surface area is several times that of the measured 2-dimensional area. Since these polymer films have porous structures it is of interest to rank/rate the performance of these films with more conventional and extensively studied porous carbon cathodes. A porous cathode of similar dimensions is required for a fair comparison.
Small carbon paste electrodes have been described that include a platinum (or other material) contact inserted into a teflon well. The well is then filled with a paste of graphite powder and mulling liquid such as mineral oil. Problems include finding a pure mulling agent which is electroinactive over the potential range of interest; completely filling the well to eliminate void; drying out; excessive pressure separating oil and carbon. Although useful in aqueous solutions, carbon paste electrodes in nonaqueous (e.g. acetonitrile, propylene carbonate) solutions tend to disintegrate. Also, they are relatively thick electrodes.
Swofford and Carman (Anal. Chem. 38, 966, 1966) reported on an electrode consisting of a carbon-epoxy resin suspension. However, the maximum amount of carbon was only 25%, and the electrode was polished to a smooth finish (low surface area). Pungor, Szepesvary and Havas (Anal. Lett. 1, 213, 1968) prepared a graphite impregnated silicone rubber electrode that was vulcanized at room temperature. The electrode was used in voltammetric analysis and the surface renewed by cutting off the end of the spent rod.
Obviously, the two previous techniques employ rigid matrices impregnated with carbon or graphite to provide electrical conductivity and an electrochemically active surface. Both methods provide a low surface area electrode and neither method enables one to observe properties of the carbon itself as a porous structure.